Every time there's a mass shooting, parents worry about their own children in school. Many of us have had similar, if not quite the same thing happen. Too many of us.
A few years ago I was sitting in my car in the school parking lot along with other parents of high schoolers who lived too far away to be able to walk (and California has no free bus service for schools) when no kids showed up. We waited and waited.
One of the parents was able to overhear police reports and discovered that several schools, including this one had received bomb threats. So the school was locked down while police searched for the possible bomb. The teens had not been told. At this point, every teen had a cell phone, so we parents were able to text messages to them to keep them from panicking. Eventually the kids were being let go, building by building, so we parents let the kids know. My kid was in building 4, so it was a while before that building was released.
Did they not tell the kids to keep them from panicking and running? Even though they were sitting quietly, they still were frantic and it was only the ability to text to their parents that kept them relatively calm.